1. Field of the Invention
The present invention relates to electrically conductive materials, more particularly, the present invention relates to an electrically conductive paste for conductive substrate or conductive film.
2. Description of Related Arts
The electrically conductive paste is made of low molecular weight liquid epoxy resin and according hardening agent in the U.S. Pat. No. 3,746,662. However, the hardening agent is flaky and hard; it is not applicable to manufacturing membrane keyboards. In the U.S. Pat. No. 4,425,263, the electrically conductive paste is made of metal powder, organic solvent and aromatic polyester resin adhesive. However, the disadvantage of the electrically conductive paste is insufficient flexibility. As a result, the membrane keyboard made of the electrically conductive paste is likely to crack. In order to enhance the flexibility of the electrically conductive paste, polyvinyl dichloride polymer is added to aromatic polyester resin in the U.S. Pat. No. 4,592,961. However, the electrically conductive paste is prone to form gel after kept in storage for a long time. In the U.S. Pat. No. 4,595,605, the soft welding electrically conductive paste is provided, where the electrically conductive paste consists of 88 to 93 wt % by weight of metal powder, and 7 to 12 wt % by weight of plastic vinyl chloride/vinyl ethylene resin polymer with total molecular weight of 14,000 to 35,000. The disadvantage of the electrically conductive paste is that the conductive cannot be attached to indium-tin oxide (ITO) films. In the U.S. Pat. No. 5,011,627, the electrically conductive paste consists of 40 to 75 wt % by weight of metal powder, 5 to 20 wt % by weight of solvent, plastic epoxy resin and phenolic resin (with molecular weight of 10,000 to 25,000). Though the flexibility of the electrically conductive paste is enhanced, but the electrically conductive paste is not as flexible as polyester membrane. In addition, the metal powder used in the electrically conductive paste is spherical, irregular and flake shaped, which delivers less conductivity in the U.S. Pat. No. 5,011,627. In the U.S. Pat. No. 5,089,173, the electrically conductive paste consists of three components polymer plastic vinyl chloride/vinyl ethylene resin/dicaboxylic acid is provided, where the electrically conductive paste consists of 3 to 15 wt % by weight of plastic vinyl chloride/vinyl ethylene resin/dicaboxylic acid and 1 to 6 wt % by weight of polyamide resin, 2 to 10 wt % by weight of polyester alkyd (or 1 to 10 wt % by weight of a mixture of polyamide resin and polyester alkyd), 0.05 to 1.0 wt % by weight of triethanolamine for neutralizing carboxyl group and 50 to 80 wt % by weight of metal powder. The electrically conductive paste is provided for membrane keyboard manufacturing. It should be noted that resin is acidic and therefore may corrode the electronic circuitry. Also, the electrically conductive paste is prone to form gel when the electrically conductive paste is kept in storage for a long time.
In the U.S. Pat. No. 5,653,918, the electrically conductive paste consists of three components polymer, plastic vinyl chloride/vinyl ethylene resin/vinyl alcohol or hydroxyl group, is provided, where the electrically conductive paste consists of 4 to 18 wt % by weight of plastic vinyl chloride/vinyl ethylene resin/vinyl alcohol or hydroxyl group, and 40 to 80 wt % by weight of metal powder. The electrically conductive paste does not have the disadvantage such as gelation, it is not ideal to use in conductive materials such as ITO film or ITO glass.